Doped gold alloy

ABSTRACT

The invention relates to an alloy of gold of at least 14 carats, characterized in containing 10 to 20,000, preferably 100 to 1000 ppm Zn, 10 to 20,000, preferably 100 to 1000 ppm Ga, 10 to 20,000 preferably 100 to 1000 ppm Ta, 10 to 10,000, preferably 90 to 950 ppm Pt, and 10 to 5000, preferably 5 to 100 ppm Ru, a cast object made from said alloy, a method for production thereof and use thereof for production of jewelry by the lost wax method.

The present invention relates to an at least 14 carat gold alloy for the manufacture of jewelry by investment casting.

Investment casting is a method for producing complex pieces having an attractive surface appearance and an excellent dimensional precision. This technique consists in firstly making a replica of each of the desired pieces from wax, by injection into tools. When assembled on casting channels, also made of wax, these models constitute a cluster; after surrounding this cluster uniformly with a ceramic shell, the wax is melted and leaves an exact impression in the ceramic, into which the molten metal is poured. After cooling, the shell is destroyed and the metal pieces are separated and finished. The use of this technique for casting gold jewelry derives from the first days of metallurgy, i.e. about 4000 years BCE. It was not until its application in dental technology, at the start of the twentieth century, that the production of models and the casting techniques which we know today were developed.

However, the various parameters of investment casting are difficult to control. Thus, it frequently arises that the pieces obtained have the following drawbacks: surface irregularity, pores due to reactions between the liquid metal and the coating (mold) leading to the release of gases, and grain refiner grouping in “pockets”. These drawbacks lead to many cast object rejects.

In “Optimizing gold alloys for the manufacturing process”, Gold Technology, 34 spring 2002, pp. 37-44, D. Ott gives a review of the various additives or dopants used to improve the properties of 14 to 18 carat yellow gold alloys based on gold-silver-copper, in particular castability, grain fineness, ductility, tensile strength and hardness. According to this article, the only elements used for this purpose in practice are zinc, silicon, iridium and cobalt.

Silicon is known to cause the formation of a stable protective oxide layer around pieces obtained by casting, when it is added in a small quantity to 14 carat gold alloys. The formation of this oxide layer makes it possible to prevent pores due to reactions between the liquid metal and the coating and to obtain a perfect surface for 9, 14 and 18 carat gold alloys. However, the addition of silicon leads to an increase in the size of the grains and a decrease in the tensile strength. These side-effects are catastrophic in the case of 18 carat gold alloys, entailing thermal instability of the alloy, an enormous grain size due to the inhibition of grain refiners, and inhibition of grain refiners.

It is an object of the invention to find dopants for an at least 14 carat gold alloy which have the advantages of silicon without presenting the aforementioned drawbacks.

This object is achieved by the invention as defined in the appended set of claims.

According to the invention, the dopants are Zn, Ga, Ta and Ru. Surprisingly, the presence of these elements makes it possible to avoid the detrimental interaction between the mold and the liquid metal during the investment casting of gold alloys, apparently owing to the formation of a gas-impermeable protective oxide layer. Ruthenium is a very effective grain refiner, even at a small level.

The invention relates to a gold alloy of at least 14 carats, characterized in that it contains as dopants, by weight, from 10 to 20,000, preferably from 100 to 1000 ppm Zn, from 10 to 20,000, preferably from 100 to 1000 ppm Ga, from 10 to 20,000, preferably from 100 to 1000 ppm Ta, from 10 to 10,000, preferably from 90 to 950, ppm Pt and from 10 to 5000, preferably from 5 to 100 ppm Ru.

The presence of these dopants at these levels makes it possible to obtain cast pieces without problems of pores and with an excellent surface condition, excellent grain size and excellent mechanical properties, in particular of pliancy and hot tensile strength. The castability of the alloy is furthermore improved.

The at least 14 carat gold alloy may be an alloy based on gold, in particular a 14 carat alloy based on gold, silver and copper, for example a 14 carat yellow gold alloy having, expressed by weight, 58-59% Au, 24-28% Ag and 13-17% Cu, or a red gold alloy having, expressed by weight, 58-59% Au, 7-11% Ag and 30-34% Cu, and 18 carat alloy, for example a yellow gold alloy having, expressed by weight, 75-76% Au, 10-14% Ag and 10-14% Cu, a pale yellow gold alloy having, expressed by weight, 75-76% Au, 14-18% Ag and 7-11% Cu, a pink gold alloy having, expressed by weight, 75-76% Au, 7-11% Ag and 14-18% Cu, a red gold alloy having, expressed by weight, 75-76% Au, 2-6% Ag and 18-22% Cu, a 22 carat alloy, for example a yellow gold alloy having, expressed by weight, 91-92% Au, 3-7% Ag and 1-5% Cu, or a red gold alloy having, expressed by weight, 91-92% Au, 0-2% Ag and 6-10% Cu.

The at least 14 carat gold alloy may also be a fine gold alloy having, expressed by weight, 99-99.9% Au and 0-1% Cu. In this case, it will expediently contain from 10 to 10,000 ppm Zn, from 10 to 10,000 ppm Ga, from 10 to 10,000 ppm Ta, from 10 to 10,000 ppm Pt and from 10 to 5000 ppm Ru.

The at least 14 carat gold alloy may also be a gray gold alloy having, expressed by weight, 75-76% Au, 8-12% Cu, 0-4% In and 11-15% Pd, or a 14 carat gray gold alloy having, expressed by weight, 58-59% Au, 14-18% Ag, 12-16% Pd and 6-10% Cu.

The same advantageous properties of the alloys are obtained when the Ta weight ratio specified is replaced by an identical weight ratio of an element selected from the group consisting of Ti, Zr and Nb.

The gold alloy according to the invention is generally manufactured as ingots by casting the constituent elements of the alloy, either in the pure state or the alloy state, under an inert atmosphere, for example nitrogen, in ingot molds made of a heat-resistant material, for example graphite. The alloy may then be shaped by continuous casting in order to obtain blocks. Continuous casting is a method in which the molten alloy is introduced into a graphite mold with open ends, in which the metal solidifies to produce a bar of predefined dimensions. The solidified shape is cooled and removed from the water-cooled mold at a controlled speed using rollers, and the material is sawed to the desired length. The blocks, which can be used directly in casting, are then obtained by cutting and marking from the bar obtained by the continuous casting.

The invention also relates to a method for manufacturing a gold alloy as defined above, which includes casting the constituent elements of the alloy, either in the pure state or in the alloy state, under an inert atmosphere.

Objects cast by the investment casting technique are generally prepared in the following way. The ingots are rolled and cut into small pieces or, if the alloy has been shaped by continuous casting, the casting blocks per se are used. The coating used consists of gypsum or silica. The wax is removed without steam at a temperature of from 140 to 160° C., then the heating cycle is as follows: holding at 200° C., rise at 5° C. per minute, holding at 650° C. for 45 minutes. The casting is then carried out by centrifuging, after melting in a graphite crucible, under nitrogen. The pieces are then released from the mold and abraded in order to remove the surface oxide. Correction and completion: generalization of the protocol actually used for preparing the cast objects.

The invention also relates to the use of the alloy defined above for the manufacture of jewelry by investment casting.

The invention also relates to a cast object comprising this alloy.

The invention will be understood more clearly with the aid of the following examples, given by way of illustration and without implying any limitation.

In these examples, the percentages are given by weight unless otherwise indicated. Furthermore, the temperature is room temperature or is expressed in degrees Celsius, and the pressure is atmospheric pressure.

All the examples furthermore form an integral part of the invention as does any characteristic of the description, including the examples, which appears to be novel vis-à-vis any prior art, this being in the form of a general characteristic rather than a particular characteristic of the example.

Reading of these examples will be facilitated by reference to FIGS. 1 and 2, and to Tables 1 and 2.

FIGS. 1 and 2 respectively represent the diagram of a wheel for evaluating the surface condition, the castability, the ductility, the porosity, the oxidation as well as the grain size of the alloy after cooling, and a photograph of a harp-shaped piece for evaluating the hot strength of the alloy.

Tables 1 and 2 respectively collate the compositions of standard and doped alloys, and the main characteristics of the molded pieces obtained from these alloys.

EXAMPLE 1 Preparation of Objects Cast Using Alloys According to the Invention

First, the alloy ingots of dimensions 80×50×5 mm³ were cast under nitrogen in graphite ingot molds, from pellets in the case of gold and silver, copper plates, thin pieces of zinc and gallium, and pre-alloys of gold-tantalum 5% and platinum-ruthenium 5% in thin sheets.

The ingots were then rolled to a thickness of 1 mm. A square plate with a side length of 2 cm was used for each alloy (after coating and polishing) for the spectrometric color analyses.

The rolled plates were then cut into pieces with a side length of about 1 cm. The coating used for the investment casting consists of gypsum and silica. The wax is removed without steam at 150° C., then the heating cycle is as follows: holding at 200° C., rise at 5° C. per minute, holding at 650° C. for 45 minutes. The casting is then carried out by centrifuging, after melting in a graphite crucible, under nitrogen. The pieces are then released from the mold and abraded in order to remove the surface oxide, then analyzed according to the procedures given below.

Table 1 specifies the composition of the cast objects for four 18 carat gold alloys according to the invention, referred to here as “doped yellow”, “doped pale yellow”, “doped pink” and “doped red”, respectively corresponding to the “standard yellow”, “standard pale yellow”, “standard pink” and “standard red” alloys obtained in example 2.

EXAMPLE 2 Preparation of Objects Cast Using Standard Alloys and Silicon-Doped Alloy

These objects were manufactured as described above, the difference being the use of gold and silver pellets and copper plates, and optionally thin pieces of zinc and silicon, when casting the ingots.

Table 1 specifies the composition of the cast objects for four 18 carat gold alloys of the prior art, referred to as “standard yellow”, “standard pale yellow”, “standard pink” and “standard red”, and a known silicon-doped alloy with a composition similar to standard yellow, referred to as “Si-yellow”.

EXAMPLE 3 Study of the Properties of the Cast Objects

The color of the alloy was measured on a square plate with a side length of 2 cm and a thickness of 1 mm, according to the three-dimensional measuring system known as CIELab, CIE standing for the Commission Internationale de l'Eclairage, and Lab standing for the three coordinate axes. The axis L measures the white-black component (black=0; white=100), the axis a measures the red-green component (red=+a, green=−a) and the axis b measuring the yellow-blue component (yellow =+b, blue=−b). For further details about this measuring system, reference may be made to the article “The Color of Gold-Silver-Copper Alloys” by R. M. German, M. M. Guzowski and D. C. Wright, Gold Bulletin 1980, 13, (3), pages 113-116. The human eye can distinguish a difference of 1 point on this scale.

The values obtained for this measurement (Table 2) show that the addition of dopants in an alloy does not have an unfavorable effect on its color.

The properties of the alloys after investment casting were evaluated for each alloy with the aid of two cast pieces. The first piece (FIG. 1) consists of a wheel on which are placed a platelet with a surface area of 1 cm² and a thickness of 1 mm as well as rods with a height of 2 cm and diameters 0.8, 0.6, 0.4 and 0.3 mm. 2 rods of each diameter are placed on the wheel, i.e. 8 rods. This first piece makes it possible to evaluate the surface condition, the castability, the pliancy, the ductility, the porosity, the oxidation as well as the grain size of the alloys after casting. The second piece in the form of a harp (FIG. 2) makes it possible to evaluate the hot strength of the alloy.

The score given to the surface condition is calculated according to the following criteria: porosity and textural fineness of the platelet. The score 10 corresponds to a perfect surface condition without any defect.

The scoring given to the surface porosity is scored by subtracting the following points from 10:

-   -   no pore or hollow visible: 0 points     -   pores and hollows visible on less than 10% of the surface: 2         points     -   pores and hollows visible on 10% to 50% of the surface: 4 points

The second criterion which relates to the fineness of the structure is scored by subtracting the following points from 10:

-   -   if the end of the platelet is straight: zero points     -   if the end of the platelet is slightly serrated: 1 point     -   if the end of the platelet is very serrated: 2 points     -   if the platelet is serrated beyond its end: 4 points     -   if the overall texture of the platelet has thin corrugations: 1         point     -   if the overall texture of the platelet has wide corrugations: 4         points.

The small relief points present on the surface are due to surface defects of the coating and are independent of the alloy, although they do impair the quality of the piece. A surface which is perfect in terms of porosity and textural fineness but has relief points will receive a score of 9.5 or 9 depending on the size and frequency of these points, in order to distinguish it from a perfect surface without relief points.

In order for the alloy to be accepted in terms of its surface, the minimum score must be 9/10, and only the defects due to the quality of the coating will be tolerated (relief points).

Table 1 shows that the alloys doped according to the invention have a satisfactory surface condition, which is improved in comparison with the corresponding standard alloys (10/7, 9/7, 9.5/7, 9/6) and identical to the silicon-doped alloy (10/10).

The various alloys were subjected to a castability test to determine the ease with which an alloy can be cast in small-diameter conduits. This property is important for the manufacture of jewelry pieces which have fine parts needing to be reproduced when casting. The score given comes from the average taken over the heights of the 8 precious alloy rods after casting. The higher the score out of 20 is, the better the castability of the alloy is.

In Table 1, the alloys doped according to the invention have a better castability than the corresponding standard alloys (14.12/9.40, 14.50/9.25, 16.90/9.40, 18.5/12.6) and the silicon-doped alloy (14.12/9.0).

The pliancy test is used to simulate the step of setting at the jeweler's. It is important that the setting rods can be folded several times so that the jeweler can make several attempts without the entire piece having to be recast. The rods folded have a diameter of 0.8 mm in this test. The pliancy test consists of a first twist through an angle of 90°, the subsequent ones being alternately opposite at angles of 180°. A value of 1 corresponds to breaking at an angle of 90°, and a value of 2 corresponds to breaking at an angle of 90°+180°. The higher values correspond to an additional twist opposite to the one before, through an angle of 180°.

Table 1 shows that the objects cast using alloys doped according to the invention have better pliancy than those made of corresponding standard alloys (4/3, 4.5/3.5, 3/2, 2/1) or made of silicon-doped alloy (4/2).

Another test (not mentioned in this table), the so-called ring enlargement test, shows that the alloys doped according to the invention are more ductile than the corresponding standard alloys and can withstand up to 24% elongation before breaking. The cast rings initially had a diameter of 15.9 mm (number 10) and a cross section of 2 mm². The standard alloy without a refiner withstands an enlargement of 2 numbers, and the doped alloy withstands an enlargement of 1.

The hot tensile strength test was carried out by casting a harp-shaped piece (FIG. 2). The difference in coefficient of expansion of the mold and the metal generates tension which can cause the metal to break depending on its fragility. This test makes it possible to discriminate the fragile structures as well as the possible detrimental contamination of the metal. The score is assigned by subtracting 1 point per broken rod from 20. Only the alloys which obtained the score 20/20 were kept.

Table 1 shows that the alloys doped according to the invention have an excellent hot tensile strength, in contrast to the silicon-doped alloy.

The state of porosity is scored by observing the platelet edge-on with an optical microscope. The score out of 10 is given as a function of the number of pores and their size and the regularity of the surface:

-   -   if large pores are observable, the score 0 is automatically         assigned     -   if pores of small size are observed at the surface (over a depth         of about 200 μm): subtract 1 or 2 points according to their         number     -   if the surface is slightly irregular: subtract 1 point     -   if pores are revealed: subtract 1 point.

More points may be taken off the score according to the severity of the problem.

The minimum acceptable score is 9/10. The pieces which have surface pores are automatically rejected. The fewer pores the alloy has, the better its mechanical properties will be and the easier it will be to polish.

Table 1 shows that the alloy doped according to the invention has a state of porosity identical to that of the silicon-doped alloy (10/10) and even better than each of the standard alloys (10/8, 10/6, 9/0 and 9/7).

The state of oxidation is scored as a function of the appearance of the piece just after release from the mold. The more the piece has a uniform appearance close to the color of the alloy, without black traces due to copper oxide, the more the score obtained will tend toward 10/10. Copper oxide should be proscribed as far as possible in the field because it does not protect the piece against the gases and it is suspected of promoting the mold degradation reactions leading to the release of sulfur dioxide gas.

The pieces obtained from the alloys doped according to the invention have a uniform surface close to the color of the alloy without copper oxide traces, and therefore have an excellent state of oxidation, even better than that of the pieces obtained from the standard alloys (Table 1: 10/0, 10/0, 10/5, 10/10).

Lastly, the ASTM grain size is given by superimposing an ASTM grid on the photograph of a metallographic grid of a cast piece after chemical attack in order to reveal the grain boundaries. According to the ASTM conversion table, a size of 7 corresponds to an average grain diameter of 32 microns. ASTM 3 in turn corresponds to an average diameter of 125 microns. The higher the ASTM value is, the smaller the grains are, the better the mechanical properties of the alloy are and the easier it will be to polish.

Table 1 shows that the pieces obtained from the alloys doped according to the invention therefore have a grain size identical to or smaller than that of the pieces obtained from the standard alloys (7/7, 7/7, 6/3-4, 6/6) or the silicon-doped alloy (7/2-3).

The results obtained in the tests reported above therefore show that adding the dopants of the invention to an 18 carat gold alloy makes it possible to improve the state of porosity, the state of oxidation, the surface condition, the pliancy and the ductility for the investment cast objects, and to preserve or decrease the grain size while maintaining the hot tensile strength and the color of the alloy. The castability of the alloy is furthermore increased, which makes it possible to manufacture jewelry pieces having fine parts. TABLE 1 Composition of the standard and doped alloys in percentages by mass weight % Au Ag Cu Ir Ta Ga Zn Pt Ru Si standard yellow 75.03 12.5 12.45 0.02 — — — — — — standard pale yellow 75.03 15.97 8.95 0.05 — — — — — — standard pink 75.03 8.94 15.98 0.05 — — — — — — standard red 75.03 4.47 20.45 0.05 — — — — — — doped yellow 75.03 12.24 12.53 — 0.05 0.05 0.05 0.0475 0.0025 — doped pale yellow 75.03 15.71 9.06 — 0.05 0.05 0.05 0.0475 0.0025 — doped pink 75.03 8.86 15.91 — 0.05 0.05 0.05 0.0475 0.0025 — doped red 75.03 4.27 20.5 — 0.05 0.05 0.05 0.0475 0.0025 — Si-yellow 75.03 9.95 13.96 0.01 1.0 0.05

TABLE 2 Comparison of the 18 carat gold alloys of standard and doped color with the various characteristics of the cast pieces. The casting conditions are rigorously the same for all the tests. The results for the silicon on-doped alloy (Si-yellow) are given by way of example. Surface Hot tensile State of State of ASTM grain Color L, a, b condition/10 Castability/20 Pliancy strength/20 porosity/10 oxidation/10 size standard yellow 93.4, 3.6, 23.5 7 9.40 3 20 8 0 7 standard pale yellow 95.3, 0.16, 25.3 7 9.25 3.5 20 6 0 7 standard pink 92.7, 5.7, 21.2 7 9.4 1 20 0 5 3-4 standard red 91.5, 8.4, 18.1 6 12.6 1 20 7 5 6 doped yellow 94.9, 3.1, 23.0 10 14.12 4 20 10 10 7 doped pale yellow 95.2, 0.14, 25.3 9 14.50 4.5 20 10 10 7 doped pink 92.3, 5.7, 21.2 9.5 16.90 3 20 9 10 6 doped red 91.1, 8.5, 17.9 9 18.5 2 20 9 10 6 Si-yellow 93.3, 3.29, 21.96 10 9.0 2 6 10 10 2-3 

1. A gold alloy of at least 14 carats, wherein it containing as dopants, by weight, from 10 to 20,000, preferably from 100 to 1000 ppm Zn, from 10 to 20,000, preferably from 100 to 1000 ppm Ga, from 10 to 20,000, preferably from 100 to 1000 ppm Ta, from 10 to 10,000, preferably from 90 to 950, ppm Pt and from 10 to 5000, preferably from 5 to 100 ppm Ru.
 2. The alloy as claimed in claim 1, which is a 14 carat alloy based on gold, silver and copper, selected from the group consisting of a yellow gold alloy having, expressed by weight, 58-59% Au, 24-28% Ag and 13-17% Cu, and a red gold alloy having, expressed by weight, 58-59% Au, 7-11% Ag and 30-34% Cu.
 3. The alloy as claimed in claim 1, which is an 18 carat alloy based on gold, silver and copper, selected from the group consisting of a yellow gold alloy having, expressed by weight, 75-76% Au, 10-14% Ag and 10-14% Cu, a pale yellow gold alloy having, expressed by weight, 75-76% Au, 14-18% Ag and 7-11% Cu, a pink gold alloy having, expressed by weight, 75-76% Au, 7-11% Ag and 14-18% Cu, and a red gold alloy having, expressed by weight, 75-76% Au, 2-6% Ag and 18-22% Cu.
 4. The alloy as claimed in claim 1, which is a 22 carat alloy based on gold, silver and copper, selected from the group consisting of a yellow gold alloy having, expressed by weight, 91-92% Au, 3-7% Ag and 1-5% Cu, and a red gold alloy having, expressed by weight, 91-92% Au, 0-2% Ag and 6-10% Cu.
 5. The alloy as claimed in claim 1, which is a fine gold alloy having, expressed by weight, 99-99.9% Au, 0-1% Cu, from 10 to 10,000 ppm Zn, from 10 to 10,000 ppm Ga, from 10 to 10,000 ppm Ta, from 10 to 10,000 ppm Pt and from 10 to 5000 ppm Ru.
 6. The alloy as claimed in claim 1, which is an 18 carat gray gold alloy having, expressed by weight, 75-76% Au, 8-12% Cu, 0-4% In and 11-15% Pd.
 7. The alloy as claimed in claim 1, which is a 14 carat gray gold alloy having, expressed by weight, 58-59% Au, 14-18% Ag, 12-16% Pd and 6-10% Cu.
 8. The alloy as claimed in claim 1 which, instead of the specified Ta weight ratio, has an identical weight ratio of an element selected from the group consisting of Ti, Zr and Nb.
 9. A cast object comprising an alloy as claimed in claim
 1. 10. A method for manufacturing a gold alloy comprising: casting a gold alloy of at least 14 carats, containing as dopants, by weight, from 10 to 20,000, preferably from 100 to 1000 ppm Zn, from 10 to 20,000, preferably from 100 to 1000 ppm Ga, from 10 to 20,000, preferable from 100 to 1000 ppm Ta, from 10 to 10,000, preferably from 90 to 950, ppm Pt and from 10 to 5000, preferably from 5 to 100 ppm Ru either in a pure state or in an alloy state, under an inert atmosphere.
 11. The method as claimed in claim 10, wherein the alloy is formed by continuous casting.
 12. Use of an alloy as claimed in of claim 1 in the manufacture of jewelry by investment casting. 